Human frontiers: How much heat can the body and mind take?

If planet-warming emissions continue to rise at their current pace, three in four people in the world will face deadly heat by the turn of the century.

What Christian Clot remembers most vividly from his days in Iran's boiling Dasht-e Lut desert was having to stay completely still for 12 hours a day - or die.

"It was so hot I had to lie down behind some rocks between 8 a.m. and 8 p.m. Staying in a tent was too dangerous as it would have instantly overheated," he recalled.

Clot, a French-Swiss explorer, is testing the limits of human endurance, including to worsening temperature extremes.

In the Iranian desert and on three other 30-day expeditions alone in the world's harshest climates, he has explored what impacts extreme weather might have on people, both physically and mentally.

"Most studies on the human body have been done in labs rather than in real settings," he told the Thomson Reuters Foundation. "I wanted to experience what you can't find in scientific journals."

If planet-warming emissions continue to rise at their current pace, three in four people in the world will face deadly heat by the turn of the century, according to a study published in the journal Nature Climate Change in June.

Emily Y.Y. Chan, a professor of public health at the Chinese University of Hong Kong, expects heatwaves to become not just more frequent but also longer by the end of the century.

That could lead to a range of worsening health problems - including some unexpected ones, such as more malnutrition.

"I knew I could die within hours of exposure to such high temperatures," he admitted.

Each day, Clot collected data, including his heart rate and body temperature, and carried out tests to assess the heat's impact on his mental abilities, including his decision-making and memory.

Although his scientific team are still analyzing the results, Clot said the biggest challenge was extreme physical and mental tiredness.

"Every movement I made was slower and demanded more effort," he explained. "I was conscious of the threat surrounding me but found it extremely challenging to stay attentive at all times."

Georges Benjamin, executive director of the American Public Health Association, thinks Clot's struggle with excessive heat is hardly unique.

"The average human doesn't tolerate heat well," he said - but some can cope better than others.

"The human body functions like the radiator of your car: your ability to cool yourself down depends on a range of factors like your age, your capacity to sweat, whether or not you're taking any medication," he said.

For those unable to regulate their body temperature effectively, the spectrum of heat-related illness ranges from simple sunburn to severe dehydration and heatstroke, which can be life-threatening, he added.

Pressure on hospitals

Another study, published in the journal Science Advances in June, found that expected future increases in temperatures globally could result in a "drastic" hike in deaths in India and other developing countries.

Separately, Chan and her team identified temperature thresholds beyond which deaths and hospital admission rates start to rise in Hong Kong.

"We found that daily mortality increases by 1.8 percent for every degree above the threshold of 28.2 degrees Celsius, while daily hospitalization – for respiratory and infectious diseases, for example – increases by 4.5 percent for every degree above the threshold of 29 degrees Celsius," she said.

She worries that governments and the public are ill-prepared to deal with rising temperatures because of a general lack of awareness about how heat can impact people's health.

Benjamin of the American Public Health Association agrees. "Human beings are terrible at evaluating risk in a pro-active way. We rationalize why not to do things," he said.

Preparedness key

To limit deaths, governments should try to understand better where the most heat-vulnerable people, such as the elderly, live so they can swiftly open emergency cooling centres nearby and boost electricity supplies when it gets too hot, Benjamin said.

In rural areas, ensuring that people have access to enough water and shelter in times of extreme heat is crucial, he said.

Chan said extreme weather warnings that take into account people's age and literacy level can help reach and protect the most vulnerable groups.

Other ways to cope with heat include adjusting the schedules of outdoor workers based on temperature, she said.

Clot said getting people to listen to, and take into account health warnings can be tough.

He plans to repeat his desert heat expedition next year, this time with a group of 10 men and 10 women. The aim is to assess how climate extremes affect group dynamics, something he hopes will help people "better adapt to weather extremes and other environmental challenges that might come our way".